The most significant factor reducing the capacity of radio systems is the limited frequency spectrum available. The capacity of a radio system is thus dependent on how efficiently the radio frequencies allocated to the system can be utilized. In cellular radio networks, enhanced utilization of radio frequencies is based on frequency reuse, the same frequency is reused at several locations that are sufficiently spaced apart, which affords a vast increase in system capacity. This is counteracted by increased complexity in the network as well as in the mobile units which must be capable of selecting a base station from among several possible base stations. For example, if the same frequency is reused in every ninth cell, the spectral allocation of N frequencies permits the use of N/9 carriers simultaneously in any cell. Diminishing the cell size or reducing the distance between cells using the same frequency will enhance capacity on the one hand, but also increases co-channel interference. Therefore, selection of the reuse factor is often a compromise between co-channel interference and the traffic carrying capacity of the system.
Since the frequency spectrum allocated to a cellular radio network is fixed and the number of subscribers is rapidly increasing, efficient use of the allocated frequency spectrum is vital to any network operator. Hence, various features increasing the traffic carrying capacity in the cellular network will provide much-needed relief to operators, particularly in crowded urban areas. Radio network evolution towards high-capacity radio networks has the following main alternatives; increasing the number of channels, splitting the cells (small cells), microcellular networks, multi-layered networks, underlay-overlay networks, and other capacity enhancement concepts, such as half-rate channels, frequency hopping, and power control. In the following, these alternatives will be described in more detail.
Increasing the Number of Channels
The simplest way to supplement capacity is by increasing the number of channels. Since the allocated cellular spectrum per network operator is very limited, this method does not give relief from capacity problems.
Splitting Cells (Small Cells)
When cell sizes are reduced below a radius of 1 km, there generally is a need to lower, the antenna height below rooftop level. This is because coverage to localized areas at street level cannot be efficiently engineered from a rooftop installation. Such lowering of antennas causes problems in designing coverage. Prediction of ranges for these types of installation is less well understood than in cases of macrocells. Furthermore, interference management becomes more difficult from below rooftop installations, as overspill into co-channel base stations cannot be equally controlled. Cell overspill may eventually reduce cell sizes to the point where conventional planning practices and radio systems do not work efficiently. Additionally, any significant capacity enhancement is accompanied by major investments in BTS sites and transmission connections. Splitting of cells is a good method for capacity relief up to a certain point. Unfortunately, urban area capacity requirements are so high that this method does not offer help in the long run. Cell splitting can therefore only be used for short term relief.
Microcellular Network
There is no exact definition of “microcellular network”. A cell having a small coverage area and antennas below rooftop level could be the characteristics in the definition of a “microcell”. Microcellular concepts are often mistakenly referred to as “multi-tiered”, but a “microcell” can be deployed without a multi-layer architecture. In implementing cell splitting below a certain limit and placing antennas below rooftop level or in buildings, advanced solution radio network planning and radio resource control is needed. An increased number of BTS sites significantly increases the costs. For cells with a radius of 300 m –1 km, signal variability due to shadow fading is very high compared to macrocells and relative to the coverage area of the small cells. These factors mean that cell overlaps need to be very high in order to meet the desired overall coverage; this is, of course, inefficient. Cells with a radius below 300 m experience more line of sight signal propagation and somewhat less signal variability, which is helpful from a coverage point of view. However, antenna location in these circumstances very significantly determines the actual coverage area. Localized blockages-which cause serious shadows effectively produce coverage holes. Small antenna location variations significantly alter the effectiveness and characteristics of the BTS site. There are two alternative solutions to these problems: to deploy more cells accepting the inefficiency of high cell overlap, or significantly increase and improve engineering effort in the actual BTS site selection and planning process. Both of these solutions increase the costs to the operator. The net result is that a microcellular network does not give a significant capacity increase without major investment in BTS sites and transmission connections.
Underlay-Overlay Network
To cope with the two conflicting goals in radio network design, i.e., coverage and capacity, it is possible to build a radio network which has two (or more) separate cell layers, one, e.g., a macrocell layer, providing overall coverage and the other, e.g., a microcell layer, providing capacity. The “coverage layer” uses a conventional frequency reuse pattern and cell range to provide seamless overall coverage. The “capacity layer” uses a very tight frequency reuse pattern and a shorter cell range to achieve high capacity with a few channels. Multi-layered networks are often also referred to as “underlay-over-lay” networks.
In an underlay-overlay network, there are many ways to control the handover between layers. The handover decision can be made on the basis of field strength or power budget values. In this case, the interference level must be predefined for each BTS site and the handover thresholds and transmit power are adjusted to minimise the interference. The interference control is always a statistical method and the resulting average quality is therefore not a quality guarantee for a single connection. For this reason, the achieved increase in capacity is questionable.